South Korea uses the code division multiple access standard for its mobile phone network. Along with the United States and Japan, South Korea is among the few countries in the world that use CDMA.
Most countries use the global system for mobile communications. In the United States, AT&T and T-Mobile are well-known networks that operate on GSM. Sprint and Verizon Wireless operate on CDMA. A U.S. cell phone operating on CDMA does not work in South Korea because the frequency bands differ. Cell phones must have tri-band or quad-band capabilities to work internationally. Phones that meet hardware requirements can use international roaming services from a U.S. carrier, which is often costly. They can also be unlocked and used with local SIM cards.